1. Field of the Invention
This invention pertains to a method and an apparatus for removing soldered electronic components from a substrate. More particularly, it relates to non-destructive, low stress removal of chips temporarily soldered to a substrate during Known-Good-Die (KGD) testing an processing of flip chip devices.
2. Description of Related Art
As the state of the art advances, devices formed on semiconductor chips become smaller in dimensions and the devices become more densely crowded on the chip. As a result there is an increasingly important need to pretest devices as a quality control measure involving identification and elimination of defective chips before use thereof. To facilitate pretesting, a technology has developed which is commonly referred to as the KGD (Known Good Die) practice in which a chip is bonded to a temporary substrate in a temporary chip attachment (TCA) process wherein, during a testing interval, the chip is mounted temporarily to a chip carrier substrate which has electrical contacts which match contacts on the chip. During the testing interval, the chip is electrically tested through the substrate. After completion of testing, the chip is removed from the substrate for future use. During the testing interval, a burn-in process is used in which the chip to be tested is bonded temporarily to the substrate. It is desirable for the bonds between the substrate and the chip to be mechanically weak, but strong enough to hold the semiconductor chip in place and to maintain good electrical connections during testing. The chip is normally positioned on the substrate so that the electrical contacts, e.g. C4 solder balls (hereinafter referred to as C4""s) on the chip are aligned with the corresponding contacts on the substrate so that the C4""s make electrical connections between the chip and the substrate.
In one process for the testing of KGD flip chip devices, silicon chips are temporarily attached by solder balls (typically a SnPb alloy, such as 3% tin (Sn) and 97% lead (Pb), referred to as 3/97 solder) to a composite or ceramic substrate (chip carrier) and subjected to electronic component testing and burn-in. Following this testing, the chips are mounted in a first fixture and subjected at ambient temperature to shear forces across the solder balls to remove the chip from the substrate. These forces typically fracture the solder balls, leaving some solder attached to the substrate and the remainder of the solder attached to the chip. Thereafter, the chips are removed from the first fixture, and those which tested as good are then mounted in a second fixture for heat processing to liquefy the solder on the chip and reform the solder balls. The resulting chips are then packaged and eventually mounted in an array of chips on another substrate.
A variety of processes and techniques have been devised and described in the art to form a temporary connection between semiconductor chips and substrates, so as to be able to readily separate the chip and the substrate after burn-in tests have been conducted. Several of these are described in U.S. Pat. No. 5,556,024 of Olson et al., commonly assigned for xe2x80x9cApparatus and Method for Removing Known Good Die Using Hot Shear Processxe2x80x9d and the teachings thereof are incorporated herein by reference. In the process of the Olson et al. patent, the substrate which is referred to as device carrier and the device are placed in a fixture, heated to the solder liquidus temperature, and the device is then pulled away from the substrate. In one embodiment, after being heated to the solder liquidus temperature, shear forces are applied sufficient to overcome solder surface tension and to separate the die and carrier. Related patents of Olson et al. include U.S. Pat. Nos. 5,707,000; 5,636,781; 5,738,267.
U.S. patent application (CA9-98-056) Ser. No. 09/460,279, filed Dec. 13, 1999, of Bergeron et al for xe2x80x9cApparatus and Method for Non-Destructive, Low Stress Removal of Soldered Electronic Componentsxe2x80x9d describes an apparatus and a method for removing circuit chips from an assembly including a one or more circuit chips attached to at least one chip carrier, or substrate. The chips are subjected to static shear with respect to the substrate, and heated to a temperature facilitating shear within a temperature range at which solder connections are solid, such that the chip is sheared off with respect to the substrate at the plane of attachment of the solder to the substrate. In addition, the chips are further heated following disassembly to a temperature at which the solder is liquid to facilitate reforming the solder for subsequent attachment of the chip into an electronic device. In addition the substrate is held within a top plate and the circuit chips are positioned within successive chip cavities within a bottom plate. Each chip cavity includes a load surface separated by a cascade effect pitch with respect to adjacent chip cavities. A cascade effect shear force is sequentially applied to the circuit chips to remove them from the substrate seriatim.
The substrate has reduced pad dimensions, which releases the C4""s therefrom, thereby permitting the C4""s to remain on the newly tested chips during the removal process so that the chips can be reattached to the final substrate in the product for which they were manufactured. In addition, the removal process is conducted at pre-reflow temperatures, i.e. below reflow temperatures. A linear shearing force is applied to the first substrate carrier by way of a single compression coil spring. This action pushes the substrate forward and forces the chip to press against a shear tab thus pre-loading the C4""s on the first chip with a known shear force. The shear force being applied is set to shear the chip when it has been heated to an elevated temperature. Therefore, at room temperature the chip remains connected to the substrate. When the loaded fixture is placed into a furnace and heated to a critical temperature, the C4 joints and the chip are sheared away from the substrate. The heating raises the temperature until it softens the C4 solder balls on all the parts until they allow the shearing force to push the first carrier forward thus shearing the first chip away from the carrier. As this carrier is moved forward by the coil spring, the first substrate makes contact to the second substrate and the remaining coil spring force is applied to the chip on the second carrier. This action continues until all chips on all carriers are sheared seriatim. With the Bergeron et al. process, the is a variation in coil spring force as the coil spring moves the first substrate forward using the Bergeron et al. process, the coil spring force rate changes. Therefore, forces applied to each chip are different. In addition, the first chip in the row is the only chip that receives the maximum shear force at room temperature which results in an inconsistent application of force.
The present invention is concerned with a cost effective device and pre-load mechanism for removing known good dies from substrate carriers. The primary advantages over existing fixtures is tooling cost, flexibility and process improvement.
An object of this invention is to provide apparatus for performing the chip removal from a substrate without a high tooling cost for machining during fabrication.
Another object of this invention is to provide such a tool while at the same time avoiding any high costs associated with adapting the tool to operate with product variations.
A very important object of this invention is to provide process consistency with uniform shear forces applied to each chip at room temperature.
The invention includes a coil spring load mechanismxe2x80x94for each carrier/chip assembly thereby allowing each shear force for each chip to be set independently.
The invention also includes a coil spring compression device which compresses the shear coil spring, positions the shearing blade to the load position and displays the force to the operator.
With this design, shear forces on each chip are monitored during the fixture load operation and the shear process for each chip is the same.
In accordance with this invention a method and apparatus are provided for separating a an assembly comprising a substrate and semiconductor device, e.g. a semiconductor chip. For example, the semiconductor device is attached to the substrate by solder connections. The assembly is to be placed into a fixture which includes a biased shearing element. First a loading force is applied which drives a biasing element such as a coil spring and the biased shearing element into a loading position for loading, i.e. placing, the assembly into the fixture. Then load/place the assembly of the substrate and the semiconductor device into a fixture. Preferably, the fixture has a window therethrough providing space for the semiconductor device. During loading the shearing element is held proximate to the semiconductor device but out of contact with therewith. Remove the loading force to permit the shearing element to contact the semiconductor device and to apply a shearing force from the shearing element to the semiconductor device. Then, upon heating the solder connections of the assembly in the fixture to a predetermined temperature, the force applied by the shearing element is sufficient to separate the semiconductor device from the substrate. Preferably the temperature is below the melting temperature of the solder at which shearing of the solder connections occurs; and the shearing element comprises a slidable blade. Alternatively, the shearing element is a blade affixed to a slidable element that is connected by a linkage to a coil spring which applies the biasing force thereto.
In accordance with another aspect of this invention removal of a circuit chip from a substrate in a fixture having a shearing element, where the chip is secured to a substrate by bonding elements is provided by applying a loading force to move a shearing element from a mechanically biased home position into an armed, loading position, preferably under a top portion of the fixture, against a biasing force proximate to a position into which the chip is to be loaded. Then load the substrate with the chip onto the fixture. Then remove the loading force to permit the armed shearing element to move to a position in which it contacts the chip and thereby applies a mechanical shearing force to the chip in response to the biasing force applied to the shearing element by the mechanical bias. Then, heat the assembly located in the fixture to a predetermined temperature, e.g. a temperature below the melting temperature of the bonding elements, until shearing of the bonding elements occurs while continuously applying the mechanical shearing force with the shearing element.
Preferably the shearing element is a blade affixed to a slidable element that is connected by a linkage to a coil spring that applies the biasing force thereto; and/or the shearing element is a blade affixed to a slidable block which is connected by a screw that is biased to a home position by a coil spring which applies the biasing force thereto.
Preferably, the shearing element is a blade affixed to a slidable block which is connected by a screw that is biased to a home position by a coil spring which applies the biasing force thereto, and an adjustable element is provided for adjusting the bias applied by the coil spring. The bonding elements comprise solder balls or the like. The solder is heated to a temperature which facilitates the shearing of the C4 joints and removal of the chips from the substrate while the solder is in a solid state.
Preferably, each substrate and the associated chip are loaded into a seat in an upper element with the chip extending through an open window through the seat. The shearing element is connected to a slide element which is connected by a screw that is biased to a home position by a coil spring which applies the biasing force thereto, and an adjustable element is provided for adjusting the biasing force applied by the coil spring. Preferably, the slide element includes a nest located below the window adapted and located for catching a chip sheared from a substrate.